Mach number for the aircraft.
3 Structures, Airfoils, and Aerodynamic Forces
CHAPTER OBJECTIVES
After completing this chapter, you should be able to:
Identify and provide examples of primary and secondary flight controls and their purpose on an aircraft.
Describe the importance of an airfoil and summarize airfoil terminology.
Identify the variables in the geometry of an airfoil and how airfoils are classified.
Characterize the development of forces on airfoils and the relationship to Bernoulli’s principle.
Determine the relationship between aerodynamic force and the center of pressure.
Compare and contrast the development of pitching moments on a symmetrical airfoil and a cambered airfoil.
Define aerodynamic center and how it changes with aircraft speed.
Review an aircraft accident summary and correlate to the importance of primary and secondary flight controls for safety of flight.
AIRCRAFT STRUCTURES
Up to this point, you may have considered the wing to be the only aerodynamically important structure of an aircraft. But in fact the entire structure of the airplane plays a role in the efficiency of an aircraft in flight, and identifying how, and to what extent, each part of an airplane structure plays a role is an important first step. We will begin with a review of the more prominent structures discussed in aerodynamics, because their direct role on lift and drag provides the foundation for more complicated discussions in the future.
Flight control systems are of two types: primary and secondary. Primary control systems on fixed‐wing aircraft include the ailerons, elevator (stabilator), and rudder and, depending on the aircraft type and aircraft speed, give the pilot a “feel” of how the aircraft is performing. Secondary systems such as trim systems, flaps, spoilers, and leading edge devices are used in relieving control pressures for the pilots, assisting primary control surfaces in high‐speed flight, or improving the performance characteristics of the aircraft in general.
Figure 3.1 Modern transport category control surfaces.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Airplanes are flown in various configurations of gear and flaps, but for this textbook we will commonly refer to clean and dirty as the two reference configurations. In a clean configuration, the gear is retracted (when applicable), and the flaps and other high‐lift devices are retracted. In the dirty configuration, the gear is considered down and locked, and the high‐lift devices are fully deployed.
Figure 3.1 shows example of airfoils found on many air transport aircraft category still in use today. Figure 3.2 details the flight control system of a helicopter, incorporating anti‐torque pedals, a cyclic, and a collective. An expanded discussion of these primary flight controls can be found in Chapter 15.
Primary Flight Controls
Ailerons
The ailerons are usually located on the trailing edge of each wing closer to the outboard area by the tip. Ailerons control roll about the longitudinal axis as they move opposite of each other when the pilot banks left or right with the yoke or stick, with the “up” aileron on the downward moving wing. Most ailerons are connected by a mechanical means to the aircraft yoke through cables, bell cranks, pulleys, and/or push‐pull tubes. Some jet aircraft have additional ailerons located in the midwing area for high‐speed maneuvering, to reduce roll rate.
Figure 3.2 Helicopter flight controls.
Source: U.S. Department of Transportation Federal Aviation Administration (2016b).
Figure 3.3 Differential ailerons.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
When the yoke/stick is moved to the left, the ailerons deflect in opposite directions, the aileron on the left wing rises and the aileron on the right wing deflects downward. This action results in more lift on the right wing and a resultant roll to the left around the longitudinal axis. Later in Section 3.3 of Chapter 4 we will explain why this happens.
There are many styles of ailerons used throughout the fixed‐wing aircraft industry today, including differential ailerons, frise‐type ailerons, and flaperons. Differential ailerons work by deflecting the up aileron more than the down aileron, increasing drag on the downward wing counteracting adverse yaw (Figure 3.3). Frise‐type ailerons project the leading edge of the raised aileron into the wind, increasing drag on the lowered wing to once again minimize the effects of adverse yaw (Figure 3.4). Flaperons combine the control surfaces of the ailerons with the function of the trailing edge flaps, as they can be lowered like flaps but still control bank angle like traditional ailerons. Chapters 12 and 13 in this textbook will discuss these items, as well as adverse yaw, in more depth regarding aircraft stability.
Figure 3.4 Frise‐type ailerons.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Elevator/Stabilator
An elevator or stabilator controls pitch about the lateral axis, allowing for varying angles of attack during flight. An elevator is attached to the trailing edge of the horizontal stabilizer, which is usually fixed to the empennage, sometimes with an angle of incidence built in. A stabilator is a one‐piece horizontal stabilizer which moves
